The present invention is concerned with apparatus including semiconductor devices such as diodes. In particular, this invention relates to integrated multiple diodes, and more particularly to resonant tunneling diodes and systems utilizing resonant tunneling diode circuits with multistable states.
A diode is a semiconductor device having a non-linear voltage-current relation. Diodes are important solid-state devices and have many electronic applications. The tunnel diode is a variety of diode with the unusual characteristic of negative differential resistance. Negative differential resistance is a voltage-current behavior where, over certain voltage ranges, increasing the voltage applied across the diode leads to decreased current carried through the diode. For certain voltage ranges, however, current flows relatively freely through the diode. In contrast, for most devices, increasing the voltage applied across the diode, within operating parameters, leads to increasing current regardless of the voltage range. Tunnel diodes, in general, have a number of applications, including high frequency oscillator circuits and high frequency electronic switches.
One type of tunnel diode is the double barrier tunnel diode, which generally includes a quantum well with thin barrier layers on either side. This structure is known as a double barrier structure and typically lies between two injection layers. The double barrier structure serves as an energy barrier to the flow of electrons that can be overcome only under certain conditions. Fulfillment of these conditions results in a negative differential resistance characteristic of interest over a range of external applied bias voltages. Electrons are injected into the double barrier structure from the conduction band of one of the injection layers under an internal electric field produced by the applied external bias voltage. The applied voltage increases the energy of the injected electrons such that they satisfy the resonant tunneling condition of the quantum barrier. Under such voltage conditions, the resonance condition is satisfied and an incoming electron has the same energy as an energy state of the quantum well. This condition enables electrons to tunnel through double barrier structure. As the bias voltage is increased further, the energy levels no longer match the energy state of the quantum well and the resonance condition is no longer satisfied. At this point the current decreases, resulting in the negative differential resistance effect.
Of particular interest are quantum well devices having current voltage characteristics including multiple negative differential resistance regions. Using traditional methods to achieve multiple negative differential resistance regions required complex circuitry. Therefore, development of a simpler, integrated circuit exhibiting multiple negative resistance regions was desirable. Such multiple regions may be obtained from a plurality of resonant states of a quantum well or from stacking several double barrier structures wells together. However, the resulting devices typically require much higher voltages corresponding to excited states as compared to the resonant voltage of a single quantum well.
In furtherance of the continuing trend towards miniaturization and increased functional density in electronic devices, much attention has been directed toward resonant-tunneling devices as characterized by operation involving a particular carrier energy coinciding with a particular quantized energy level in a potential well. Extensive literature has been developed surrounding both the theoretical and practical device aspects as surveyed, e.g., by:
F. Capasso et al., xe2x80x9cResonant Tunneling Through Phenomena . . . in Superlattices, and Their Device Applicationsxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. QE-22 (1986), pp. 1853-1869.
Also, noting that resonant-tunneling devices may be made as diodes and as transistors; see, e.g.,
E. R. Brown et al., xe2x80x9cMillimeter-band Oscillations Based on Resonant Tunneling in a Double-barrier Diode at Room Temperaturexe2x80x9d, Applied Physics Letters, Vol. 50 (1987), pp. 83-85;
H. Toyoshima et al., xe2x80x9cNew Resonant Tunneling Diode with a Deep Quantum Wellxe2x80x9d, Japanese Journal of Applied Physics, Vol. 25 (1986), pp. L786-788;
H. Morkoc et al., xe2x80x9cObservation of a Negative Differential Resistance Due to Tunneling through a Single Barrier into a Quantum Wellxe2x80x9d, Applied Physics Letters, Vol. 49 (1986), pp. 70-72;
F. Capasso et al., xe2x80x9cResonant Tunneling Transistor with Quantum Well Base and High-energy Injection: A New Negative Differential Resistance Devicexe2x80x9d, Journal of Applied Physics, Vol. 58 (1985), pp. 1366-1368;
N. Yokoyama et al., xe2x80x9cA new Functional, Resonant-Tunneling Hot Electron Transistor (RHET)xe2x80x9d, Japanese Journal of Applied Physics, Vol. 24 (1985), pp. L853-L854;
F. Capasso et al., xe2x80x9cQuantum-well Resonant Tunneling Bipolar Transistor Operating at Room Temperaturexe2x80x9d, IEEE Electron Device Letters, Vol. EDL-7 (1986), pp. 573-575;
T. Futatsugi et al., xe2x80x9cA Resonant-Tunneling Bipolar Transistor (RBT): A Proposal and Demonstration for New Functional Devices with High Current Gainsxe2x80x9d, Technical Digest of the 1986 International Electron Devices Meeting, pp. 286-289;
T. K. Woodward et al., xe2x80x9cExperimental Realization of a Resonant Tunneling Transistorxe2x80x9d, Applied Physics Letters, Vol. 50 (1987), pp. 451-453;
B. Vinter et al., xe2x80x9cTunneling Transfer Field-effect Transistor: A Negative Transconductance Devicexe2x80x9d, Applied Physics Letters, Vol. 50 (1987), pp. 410-412;
A. R. Bonnefoi et al., xe2x80x9cInverted Base-collector Tunnel Transistorsxe2x80x9d, Applied Physics Letters, Vol. 47 (1985), pp. 888-890;
S. Luryi et al., xe2x80x9cResonant Tunneling of Two-dimensional Electrons through a Quantum Wire: A Negative Transconductance Devicexe2x80x9d, Applied Physics Letters, Vol. 47 (1985), pp. 1347-1693; and
S. Luryi et al., xe2x80x9cCharge Injection Transistor Based on Real-Space Hot-Electron Transferxe2x80x9d, IEEE Transactions on Electron Devices, Vol. ED-31 (1984), pp. 832-839.
Of particular interest are integrated devices having multiple negative differential resistance current-voltage characteristics in order to obtain circuits with multistable states. Multiple resonant tunneling diodes in series are a well-known circuit component for supplying multistable states for digital logic and signal processing. The total voltage can be distributed across the circuit elements in more than one way, depending on the history of the circuit, thus defining the multistable states. For some circuit applications it is preferable to control the total current instead of the total voltage. Also, the total of the voltages across the series of resonant tunneling diodes add together and can become too high for convenient processing by the rest of the circuit. It is natural to then consider a pair of resonant tunneling diodes in a parallel arrangement instead of in series. The most efficient way to do this is to grow one resonant tunneling diode epitaxial structure and then divide it up electrically using lithography. However, two such identical resonant tunneling diodes in parallel are basically equivalent to one resonant tunneling diode with a combined area of the two, unless something is done to differentiate them. The I(V) curves of the two or more resonant tunneling diodes must have different shapes, i.e., the peak and valley voltages differing, not just by a scale factor on the current. Then, they will naturally not behave similarly under the same bias. For example, a given bias voltage might put one resonant tunneling diode near its peak current while the other was near its valley current.
An example of previous methods for obtaining multiple stable solutions by putting two resonant-tunneling diodes in parallel is that of Capasso et al., U.S. Pat. No. 4,902,912 in which the two resonant-tunneling diodes are separated by a resistance consisting of a low doped region between the resonant-tunneling diodes. FIG. 1(a) provides a basic circuit diagram of this concept. Although the current out of the contact exhibits double peaks and multiple stable states for certain voltage differences, the required use of multiple voltage sources increases the complexity of the device. Additionally, the device is not easily adjustable or controllable. An attempt to improve on the concept developed by Capasso et al. was made by J. Soderstrom and T. Anderson, as discussed in Electron Device Letters, Vol. 9, No. 5, May 1988. FIG. 1(b) provides a basic circuit diagram of this concept. Soderstrom and Anderson introduced two resistors in series with the resonant-tunneling diodes. By choosing the resistors properly, they caused the circuit to produce a double peak curve. However, this concept has the disadvantage that the resistors introduce hysteresis, which causes the presence of different peak voltages for forward and backward voltage sweeps, and also decreases the speed of the device due to the RC time constant. The structure of Soderstrom and Anderson use lightly doped regions in order to vary the resistance for each of the diodes.
Accordingly, it is an object of the present invention to provide an integrated circuit including laterally varying diodes with multiple current-voltage characteristics that can be tailored for multiple stable solutions. Specifically, the diodes utilized for demonstration here are of the resonant tunneling type. It is another object of the present invention to provide apparatus in accordance with the invention, which give examples of its usage.
The present invention provides a plurality of laterally varying diodes and a method for producing them. The plurality of laterally varying diodes includes, in common, an nxe2x88x92 collector region formed on a diode region, with the nxe2x88x92 collector region having a contact surface opposite the diode region and a depth extending from the contact surface to the diode region. Each of the individual diodes includes an independently selectable portion of the depth with an ion-implanted portion. The plurality of laterally varying diodes further including means for substantially electrically isolating each individual diode.
The method for producing a plurality of laterally varying diodes includes the steps of providing a wafer having a diode region and a nxe2x88x92 collector region having a depth; masking the wafer to isolate the effects of ion-implantation to the desired portion of the nxe2x88x92 collector region; ion-implanting the desired portion of the nxe2x88x92 collector region to a desired depth; repeating the masking and ion-implantation steps a desired number of times to produce a desired number of ion-implanted portions in the nxe2x88x92 collector region; and providing means for electrically isolating the individual portions of the diode region and the nxe2x88x92 collector region corresponding to the ion-implanted portions in the nxe2x88x92 collector region.
A second method for producing a plurality of laterally varying diodes includes the steps of providing a wafer having a diode region and a nxe2x88x92 collector region having a depth; masking a portion of the nxe2x88x92 collector region; etching the non-masked portion of the nxe2x88x92 collector region to desired depth; repeating the masking and etching steps a for a desired number of portions of the nxe2x88x92 collector region; uniformly ion-implanting the desired nxe2x88x92 collector regions; and providing means for electrically isolating the individual portions of the diode region and the nxe2x88x92 collector region corresponding to the ion-implanted portions in the nxe2x88x92 collector region.
The present invention may be used to develop a variety of diode types. It is discussed herein in terms of the provision of a plurality of laterally varying resonant tunneling diodes.